LNBs for satellite broadcast receivers have been developed. An LNB receives a polarized signal supplied from a satellite, and converts the polarized signal into a signal in an IF frequency band (950 MHz-2150 MHz).
FIG. 11 outlines a satellite broadcast receiver 101. The satellite broadcast receiver 101 includes an LNB 102, a tuner 103, and a tuner 104. The satellite broadcast receiver 101 is connected to an antenna 100 and a TV receiver 105.
How the components of the satellite broadcast receiver 101 operate is briefly described. The LNB 102 receives, from the antenna 100, the polarized signal in a zero-th frequency band (12.2 GHz-12.7 GHz), and outputs a signal in the IF frequency band. This will be described in detail later.
The tuners 103 and 104 perform a channel selection process so as to extract, from the signal supplied from the LNB 102, a frequency component of the channel designated by the user. The tuners 103 and 104 also perform a decoding process so as to decode a video signal and an audio signal based on the signal selected in the channel selection process.
The TV receiver 105 receives, from the tuner 103 or 104, the video signal and audio signal, and displays the program of the channel designated by the user.
Details of the LNB 102 will be given. The LNB 102 receives M (M≧2) types of polarized signals from N (N≧2) satellites. This LNB is disclosed in the Patent Document 1 (Japanese Laid-Open Patent Application No. 2004-350149; published on Dec. 9, 2004). It is assumed in the present case that the LNB receives two types of polarized signals from two satellites (first and second satellites), respectively.
FIG. 12 shows the circuit configuration of the LNB 102. As shown in the figure, the LNB 102 includes input terminals P11, P12, P21, and P22, low noise amplifiers 3A, 3B, 4A, and 4B, image rejection filter circuits 5A, 5B, 6A, and 6B, local oscillators 13 and 14, frequency conversion circuits 30A and 30B, signal couplers 11A and 11B, a signal recombination circuit 55, a microcomputer 16, intermediate frequency amplifiers 17A and 17B, capacitors 18A and 18B, output terminals 20A and 20B, and a power supply circuit 22.
The following will describe how the components of the LNB 102 operate.
The input terminals P11 and P12 correspond to the first satellite. The input terminal P11 receives the first polarized signal (left-hand polarized signal) of the first satellite, whereas the input terminal P12 receives the second polarized signal (right-hand polarized signal) of the first satellite. The input terminals P21 and P22 correspond to the second satellite. The input terminal P21 receives the first polarized signal (left-hand polarized signal) of the second satellite, whereas the input terminal P22 receives the second polarized signal (right-hand polarized signal) of the second satellite.
The low noise amplifiers 3A and 4A correspond to the first satellite. The low noise amplifier 3A subjects the first polarized signal of the first satellite to low noise amplification, whereas the low noise amplifier 4A subjects the second polarized signal of the first satellite to low noise amplification. The low noise amplifiers 3B and 4B correspond to the second satellite. The low noise amplifier 3B subjects the first polarized signal of the second satellite to low noise amplification, whereas the low noise amplifier 4B subjects the second polarized signal of the second satellite to low noise amplification.
The image rejection filter circuits 5A and 6A correspond to the first satellite. The image rejection filter circuit 5A rejects an image signal of the first polarized signal having been subjected to the low noise amplification by the low noise amplifier 3A, whereas the image rejection filter circuit 6A rejects an image signal of the second polarized signal having been subjected to the low noise amplification by the low noise amplifier 4A. The image rejection filter circuit 5B and 6B correspond to the second satellite. The image rejection filter circuit 5B rejects an image signal of the first polarized signal having been subjected to the low noise amplification by the low noise amplifier 3B, whereas the image rejection filter circuit 6B rejects an image signal of the second polarized signal having been subjected to the low noise amplification by the low noise amplifier 4B.
The local oscillators 13 and 14 generate sinusoidal signals (local oscillator signals) at 11.25 GHz and 14.35 GHz, respectively.
The frequency conversion circuit 30A corresponds to the first satellite and includes mixers 7A and 8A, a high-pass filter 9A, and a low-pass filter 10A. The frequency conversion circuit 30A converts, into the IF frequency band, the frequency bands of the first and second polarized signals of the first satellite, without the overlapping of the frequency bands.
More specifically, in the mixer 7A, the first polarized signal of the first satellite, which is supplied from the image rejection filter circuit 5A, is mixed with the local oscillator signal at 14.35 GHz, which is generated as a result of the oscillation of the local oscillator 14. The mixer 7A then outputs the first polarized signal of the first satellite, whose frequency is within the first IF frequency band (1650 MHz-2150 MHz), and the signal passes through the high-pass filter 9A.
In the mixer 8A, the second polarized signal of the first satellite, which is supplied from the image rejection filter circuit 6A, is mixed with the local oscillator signal at 11.25 GHz, which is generated as a result of the oscillation of the local oscillator 13. The mixer 8A then outputs the second polarized signal of the first satellite, whose frequency is within the second IF frequency band (950 MHz-1450 MHz), and the signal passes through the low-pass filter 10A.
The frequency conversion circuit 30B corresponds to the second satellite, and includes mixers 7B and 8B, a high-pass filter 9B, and a low-pass filter 10B. The frequency conversion circuit 30B converts, into the IF frequency band, the frequency bands of the first and second polarized signals of the second satellite, without the overlapping of the frequency bands.
More specifically, in the mixer 7B, the first polarized signal of the second satellite, which is supplied from the image rejection filter circuit 5B, is mixed with the local oscillator signal at 14.35 GHz, which is generated as a result of the oscillation of the local oscillator 14. The mixer 7B then outputs the first polarized signal of the second satellite, whose frequency is within the first IF frequency band, and the signal passes through the high-pass filter 9B.
In the mixer 8B, the second polarized signal of the first satellite, which is supplied from the image rejection filter circuit 6B, is mixed with the local oscillator signal at 11.25 GHz, which is generated as a result of the oscillation of the local oscillator 13. The mixer 8A then outputs the second polarized signal of the first satellite, whose frequency is within the second IF frequency band, and the signal passes through the low-pass filter 10B.
The signal coupler 11A corresponds to the first satellite. The signal coupler-11A subjects, to frequency multiplication, the first polarized signal having passed through the high-pass filter 9A and the second polarized signal having passed through the low-pass filter 10A, so as to output a first synthesized signal. Therefore, the first synthesized signal is arranged in such a manner that, the first polarized signal of the first satellite, which is included in the first IF frequency band, is provided on the high-band side, whereas the second polarized signal of the first satellite, which is included in the second IF frequency band, is provided on the low-band side.
The signal coupler 11B corresponds to the second satellite. The signal coupler 11B subjects, to frequency multiplication, the first polarized signal having passed through the high-pass filter 9B and the second polarized signal having passed through the low-pass filter 10B, so as to output a second synthesized signal. Therefore, the second synthesized signal is arranged in such a manner that, the first polarized signal of the second satellite, which is included in the first IF frequency band, is provided on the high-band side, whereas the second polarized signal of the second satellite, which is included in the second IF frequency band, is provided on the low-band side.
In response to an instruction of the microcomputer 16, the signal recombination circuit 55 selects, for one outputs, two signals from the first and second synchronized signals. Overlapping of the selection is permitted. Then the signal recombination circuit 55 fetches two polarized signals from the polarized signals in the selected two signals, and synthesizes the fetched polarized signals and outputs the result of the synchronization.
Details of the signal recombination circuit 55 will be given in reference to FIG. 13.
As shown in the figure, the signal recombination circuit 55 includes a 2×4 switch circuit 33 and a bandwidth conversion/synchronization circuit 60. Input terminals I1 and I2 are those of the signal recombination circuit 55. Output terminals O1 and O2 are those of the signal recombination circuit 55. The bandwidth conversion/synchronization circuit 60 includes a local oscillator 47, frequency control circuits 56A, 56B, 56C, and 56D, low-pass filters 36A and 36B, high-pass filters 37A and 37B, and signal couplers 38A and 38B.
The 2×4 switch circuit 33 operates in the aforesaid IF frequency band. In response to an instruction from the microcomputer 16, the 2×4 switch circuit 33 outputs, to the respective terminals M1-M4, the first synchronized signal supplied from the signal coupler 11A and the second synchronized signal supplied from the signal coupler 11B. The number of combinations of these signals is 24.
The local oscillator 47 generates a local oscillator signal at 3.1 GHz.
The frequency control circuits 56A, 56B, 56C, and 56D convert the frequency of the first or second synchronized signals supplied from the terminals M1-M4.
The frequency control circuit 56A is provided with: a three-terminal switch 34A switched by the microcomputer 16; and a mixer 35A on the path which is selected when the switch 34A is ON. The path which is selected when the switch 34A is OFF is a bypass connected to the output side of the mixer 35A.
The frequency control circuit 56A operates as follows: when the frequency conversion of the first and second synchronized signals is carried out, the switch 34A is turned ON and the mixer 35A carries out the frequency conversion; meanwhile, when the frequency conversion of the first and second synchronized signals is not carried out, the switch 34A is turned OFF and the signals pass through the bypass. Details of these operations will be given below.
When the switch 34A is OFF, i.e. when the frequency conversion of the first and second synchronized signals is not carried out, the first or second synchronized signal supplied from the terminal M1 passes through the bypass. On this account, the output signal of the frequency control circuit 56A is the aforesaid first or second synchronized signal. That is, provided on the high-band side of the output signal of the frequency control circuit 56A is the first polarized signal, of the first or second satellite, which is on the high-band side of the first or second synchronized signal. Provided on the low-band side is the second polarized signal, of the first or second satellite, which is on the low-band side of the first or second synchronized signal. Thereafter, the second polarized signal of the first or second satellite, which is on the low-band side of the output signal of the frequency control circuit 56A, is supplied to the signal coupler 38A via the low-pass filter 36A.
When the switch 34A is ON, i.e. when the frequency conversion of the first and second synchronized signals is carried out, the first or second synchronized signal supplied from the terminal M1 is mixed, in the mixer 35A, with the local oscillator signal at 3.1 GHz which is supplied from the local oscillator 47, so that the frequency of the first or second synchronized signal is converted. That is, provided on the high-band side of the output signal of the frequency control circuit 56A is the second polarized signal, of the first or second satellite, which is on the low-band side of the first or second synchronized signal. Provided on the low-band side of the output signal of the frequency control circuit 56A is the first polarized signal, of the first or second satellite, which is on the high-band side of the first or second synchronized signal. Thereafter, the first polarized signal, of the first or second satellite, which is on the low-band side of the output signal of the frequency control circuit 56A, is supplied to the signal coupler 38A via the low-pass filter 36A.
The frequency control circuit 56B includes: a three-terminal switch 34B switched by the microcomputer 16; and a mixer 35B on the path selected when the switch 34B is ON. The path selected when the switch 34B is OFF is the bypass which is connected to the output side of the mixer 35B.
The frequency control circuit 56B operates as follows: when the frequency conversion of the first and second synchronized signals is carried out, the switch 34B is turned ON and the mixer 35B carries out the frequency conversion; meanwhile, when the frequency conversion of the first and second synchronized signals is not carried out, the switch 34B is turned OFF and the signals pass through the bypass. Details of these operations will be given below.
When the switch 34B is OFF, i.e. when the frequency conversion of the first and second synchronized signals is not carried out, the first or second synchronized signal supplied from the terminal M2 passes through the bypass. Therefore, the output signal of the frequency control circuit 56B is identical with the aforesaid output signal of the frequency control circuit 56A when the switch 34A is OFF. Thereafter, the first polarized signal, of the first or second satellite, which is on the high-band side of the output signal of the frequency control circuit 56B, is supplied to the signal coupler 38A via the high-pass filter 37A.
When the switch 34B is ON, i.e. when the frequency conversion of the first and second synthesized signals is carried out, the first or second synchronized signal supplied from the terminal M2 is mixed, in the mixer 35B, with the local oscillator signal at 3.1 Ghz supplied from the local oscillator 47, so that the frequency of the first or second synchronized signal is converted. As a result, the output signal of the frequency control circuit 56B is identical with the aforesaid output signal of the frequency control circuit 56A when the switch 34A is ON. Thereafter, the second polarized signal, of the first or second satellite, which is on the high-band side of the output signal of the frequency control circuit 56B, is supplied to the signal coupler 38A via the high-pass filter 37A.
The signal coupler 38A synthesizes the aforesaid signal having passed through the low-pass filter 36A and the aforesaid signal having passed through the high-pass filter 37A, so as to output a third synthesized signal.
The frequency control circuit 56C is provided with: a three-terminal switch 34C switched by the microcomputer 16; and a mixer 35C on the path selected when the switch 34C is ON. The path selected when the switch 34C is OFF is the bypass connected to the output side of the mixer 35C. Since the frequency control circuit 56C operates in the same manner as the frequency control circuit 56A, the description thereof is omitted.
The frequency control circuit 56D is provided with: a three-terminal switch 34D switched by the microcomputer 16; and a mixer 35D on the path selected when the switch 34D is ON. The path selected when the switch 34D is OFF is the bypass connected to the output side of the mixer 35D. Since the frequency control circuit 56D operates in the same manner as the frequency control circuit 56B, the description thereof is omitted.
The signal coupler 38B synthesizes a signal having passed through the low-pass filter 36B and a signal having passed through the high-pass filter 37B, so as to output a fourth synthesized signal.
The intermediate frequency amplifier 17A amplifies the third synthesized signal, whereas the intermediate frequency amplifier 17B amplifies the fourth synthesized signal.
The capacitor 18A removes a low frequency noise of the third synthesized signal amplified by the intermediate frequency amplifier 17A. The capacitor 18B removes a low frequency noise of the fourth synthesized signal amplified by the intermediate frequency amplifier 17B.
As a result of the above, the output terminals 20A and 20B output the third and fourth synthesized signals each of which is arranged such that any two of the first polarized signal of the first satellite, the second polarized signal of the first satellite, the first polarized signal of the second satellite, and the second polarized signal of the second satellite are provided on the high-band side and the low-band side, respectively. The power supply circuit 22 supplies power to the components of the LNB 102.
As described above, in the frequency control circuits 56A, 56B, 56C, and 56D, a case where the frequency conversion of the input signal supplied to the signal recombination circuit 55 is carried out and a case where the frequency conversion is not carried out is switched over by switching the path through which the input signal passes. The switching of the path is carried out by turning ON/OFF the switches 34A, 34B, 34C, and 34D in each of the frequency control circuits 56A, 56B, 56C, and 56D.
In the foregoing arrangement, however, when the switch 34A, 34B, 34C, or 34D is switched ON/OFF, the input signal is leaked to the path which is not connected to the switch 34A, 34B, 34C, or 34D, because of the parasitic capacity of that path.
More specifically, when the switch 34A, 34B, 34C, or 34 D is OFF, the input signal is leaked to the path to which the mixer 35A, 35B, 35C, or 35D is connected, on account of the parasitic capacity on the path to which the mixer 35A, 35B, 35C, or 35D is connected. The leakage of the input signal, however, is sufficiently attenuated when passing through the mixer 35A, 35B, 35C, or 35D.
On the other hand, when the switch 34A, 34B, 34C, or 34D is ON, the input signal is leaked to the bypass which is the path selected when the switch 34A, 34B, 34C, or 34D is OFF, because of the parasitic capacity on the bypass. Being different from the case where the switch 34A 34B, 34C, or 34D is OFF, in this case the input signal having been subjected to the frequency conversion in the mixer 35A, 35B, 35C, or 35D is mixed with the leakage of the input signal passing through the bypass, on the output side of the mixer 35A, 35B, 35C, or 35D. For this reason, the quality of the input signal is deteriorated.